1. Field of the Invention
This invention relates to a method of forming a zinc sleeve on an insulator pin.
2. Related Art Statement
Insulators such as suspension insulators are generally coupled in series so as to form insulator strings, and ground side ends of such insulator strings are suspended from arms of transmission line towers so as to hold line conductors at the opposite ends thereof. The insulator strings provide electrical insulation between the line conductor and the ground. However, when the insulator surfaces are contaminated and moistened, a leakage current flows along the surfaces of the insulating bodies of the insulators such as insulating porcelain bodies. The leakage current tends to cause electrolytic corrosion of metallic insulator pins (to be referred to as insulator pins, hereinafter) of the insulators. If such corrosion occurs inside the cement which bonds the insulator pin to the insulator body, it may crack the insulator body. On the other hand, if such corrosion occurs on the insulator pin outside the insulator body, the corrosion current tends to remove metal from the insulator pin to reduce its diameter, and in the worst case, the insulator pin may be broken at the corroded portion by the load of the line conductors.
To solve the electrolytic corrosion problem of the insulator pin, a zinc sleeve has been used. Referring to FIG. 4, a typical insulator pin 11 of a suspension insulator is firmly secured to an insulator body 8 by burying its enlarged end portion 11a in the inside of a cylindrical closed-top core 8a of the insulator body 8 by cement 9. An enlarged coupling portion 11b is formed at the opposite end of the insulator pin 11, which portion 11b fits in the socket hole of a metal cap 10 of another insulator so as to facilitate connection of the insulators. In addition to the enlarged end portions 11a and 11b, a large-diameter zinc sleeve 11c is formed at an intermediate portion of the insulator pin 11 as an integral part thereof, so as to minimize the burning or the metal-removing effects of the above-mentioned leakage current.
FIG. 3 shows a typical conventional method for making the zinc sleeve 1c. A die 2 with molding cavities 2b is separable into two parts, and it is placed on a support table 1. The die 2 has through holes 2a, each of which is concentric with the corresponding cavity 2b, and separately prepared insulator pins 11 are inserted in the holes 2a, respectively. The enlarged end portion 11a of each insulator pin 11 is supported by a holder 4. Molten zinc 5 is poured into the molding cavities 2b through a pouring basin 2c and the gate holes 2d of the die 2, so as to cast the zinc sleeves 11c in the molding cavities 2b.
The above conventional method to form the zinc sleeve 11c, however, has the following two major problems. The first problem is that the presence of the pouring basin 2c and the gate holes 2d results in formation of extra projections on the zinc sleeves 11c. To finish the insulator pins 11 after the molding of the zinc sleeves 11c thereon, additional work of cutting off such projections and polishing the cut surfaces is required. Further, the amount of molten zinc necessary for the formation of the zinc sleeves 11c is increased by an amount corresponding to the volumes of the pouring basin 2c and the gate holes 2d, so that losses of zinc material and heating energy increase accordingly. Due to the need of extra finishing work and the increased losses, the conventional method of forming the zinc sleeves 1c is costly.
The second problem is in that the probability of forming small cavities within the metal of the zinc sleeve 11c is high, because both the temperature T.sub.m of the die 2 and the temperature T.sub.p of the insulator pin 11 are low when the molten zinc is poured in the molding cavity 2b and the cooling and solidification of the molten zinc in the cavities 2b proceed both from the outer surface and inner surface of the annular sleeve 11c toward the inside thereof. Thus, the yield rate of a good product by the conventional method is rather low, e.g., about 30%.